Generally in a bar code reader the bar code is scanned so as to provide a time sequential optical image of the bar code and this image is then converted into an electrical signal and decoded. The scanning optics may be active or passive, that is to say a scanning beam may actively illuminate a bar code which is then read off the reflected beam or a scanning beam may act to take a time sequential optical image off a bar code having its own continuous illumination such as daylight or some form of conventional artificial illumination. The present invention is concerned with both active and passive systems.
In the simplest scanning geometry, only a single scanning line is required. The scanner is then very compact and hats been applied widely in hand-held bar code readers.
Because of the need for a large longitudinal working range and high spatial resolution, the laser beam has popularly been used for bar code reading. In most laser scanners, this beam is steered by a reflective means, such as a mirror, to the bar code. The reflective means can be facets of rotation of a polygonal mirror, driven by a conventional or an oscillation motor.
In order to achieve low power consumption, large scanning angle and compact dimensions, resonant scanning is very attractive for bar code reading. Together with mechanical elastic components, the mirror can be forced to oscillation under an alternating torque. When the resonant conditions are satisfied, the power consumption of the system will be extremely low. A bar code reader, in which an oscillating mirror with mechanical elastic components such as springs, is driven by a magnet-coil device, has been described in the European Patent Application 0,456,095. However, in practice of bar code reading, the large scan angle and high scan frequency (ratio) have been required. It leads to the failure of the mechanical elastic components and the shortage of the life time of the reader device.
The mirror can also be driven by a ferro-electric component. In order to increase the amplitude, a penta-bimorph structure has been used, such as is described in U.S. Pat. No. 4,387,297. However, this structure still offers very limited scanning angle. It is still difficult to be used directly in a bar code scanner.
Other prior art systems are disclosed in: EP 0063934 A2 (SONY), EP 0143483 A1 (PHILIPS), GB 2070832 A (SYMBOL), U.S. Pat. No. 4,664,476 A (KASUGA), U.S. Pat. No. 4,778,232 A (IRBY), U.S. Pat. No. 4,828,347 A (KEISER), U.S. Pat. No. 5,097,356 A (PAULSEN) and U.S. Pat. No. 5,107,372 A (GELBART).
The scan frequency in most of the above-mentioned scanners depend on the elastic properties of a mechanical component of the resonant system. It can be changed only if the mechanical properties of the elastic and the vibration components, such as spring and mirror, are changed. It means that only if the spring and or the mirror is replaced, can the resonant frequency be changed and the scan ratio then be modified.
On the other hand, aging and variation in environmental conditions may lead to variation in the elastic properties of the mechanical components. It leads to a possible shift of the resonant frequency, and the scan angle can then decrease if the driving frequency does not change and the system is therefore no longer acting at resonance.
In the present invention, using a new form of resonant scanning system, we present a new device which avoids using conventional mechanical elastic components. Based upon these devices, compact bar code readers can be produced which offer a large scan angle and high scan ratio or frequency.